Abstract
Coloration factor and coloration mechanism of the Yudamari crater lake on Mt. Aso and the Yugama crater lake on Mt. Shirane, which are typical hyper-acid crater lakes in Japan and that were observed to be cloudy emerald green in color, are discussed using data from color measurements, chemical analyses of lake waters and the grainsize distribution of suspended elemental sulfur. The grain-size distribution of elemental sulfur in the crater lakes, which is formed by the reaction of SO2(ap) and H2S(ap) originating from the bottom fumaroles, was seen to agree with that of a synthetic colloidal sulfur solution when it turned bluish white. The existence of colloidal sulfur particles involved in Rayleigh scattering and Mie scattering was confirmed. Color measurements using a colorimeter showed that the coloration of the crater lakes is not only caused by the scattering of sunlight by colloidal sulfur particles, but also by the absorption of sunlight by dissolved ferrous ion.
